(1) Field of the Invention
This invention relates to aluminum thin plates for fins of condensers and the like which are excellent in sagging resistance at high temperature and which are suitable for non-corrosive flux brazing and carrier gas brazing, and a method for preparing the same.
(2) Description of the Prior Art
Heretofore, heat exchangers made from aluminum alloys which can be used as condensers for automotive coolers, evaporators and the like have been constituted by extruded multi-hole tubes (perforated tubes) and corrugated fins.
In general, the extruded multi-hole tube may be manufactured from A1050 (According to Japan Industrial Standard. The same rule applies hereinafter) which contains 99.5 wt % or more of Al (the unit "wt %" will be hereinafter and in claims referred to simply as "%") or an A3003 alloy (Al-0.15% Cu-1.1% Mn), and the corrugated fin may be manufactured from a so-called brazing sheet in which A3003 or A3203 (Al-1.1% Mn) as a core material is clad on both the surfaces thereof with an Al-Si alloy such as A4343 (Al-7.5% Si) or A4004 (Al-10% Si-1.5% Mg) as a brazing material.
Usually a standard cladding ratio of the brazing material on the core material is within the range of 8 and 13% on either surface thereof, and a total thickness of each fin plate is about 0.16 mm.
The heat exchanger in which the brazing sheets are employed may generally be manufactured in accordance with a so-called brazing method of heating the extruded multi-hole tubes and the brazing sheets up to a temperature of 590.degree.to 620.degree. C. for several minutes in order to bond them. In this case, for the purposes of disrupting oxide layers on the surfaces of the brazing material and accelerating the flow of a solder, two methods are mainly put into practice, and one is a furnace brazing process in which a flux consisting of chloride is employed and another is a vacuum brazing process in which an Mg evaporation effect (a getter function) is utilized.
As a measure to reduce manufacturing cost of the heat exchangers, it can be conceived to make their materials thin and light, that is, in this case, to render the tubes and fins thin. With regard to the tubes, their wall thickness has been from about 1 to about 1.2 mm in former days, but owing to subsequent researches, the tubes having a wall thickness of about 0.5 to about 0.6 mm are put into practice in recent years.
However, with regard to the fins, their wall thickness remains unchanged, and thus it is within the range of 0.16 to 0.2 mm. In short, the minimum wall thickness of the fins is still on the level of 0.16 mm. The reason for this is as follows: In manufacturing the heat exchanger, the fins are heated at a high temperature of 590.degree.to 620.degree. C., as described above, and as a result, a brazing material clad on the surfaces of the fins is melted. And the melted solder is further diffused up to the core material of the fins, so that the high-temperature strength of the fins falls down, with the result that the fins cause sagging, in other words, a so-called sagging phenomenon occurs. For this reason, the conventional brazing sheets are not satisfactory in point of the wall thickness.
As for the brazing sheets used in the non-corrosive flux brazing process using a fluoride containing flux or the carrier gas brazing process of brazing techniques, it is necessary to add Zn to such a core material of the corrugated fins (or both the core material and the brazing material as the skin material) as the A3003 alloy in order to provide the core material with an effect as sacrificial anodic fins and to thereby protect the extruded tube material from corrosion. However, the addition of Zn lowers the high-temperature sagging resistance, though the sacrificial anodic effect can be enhanced.
Further, in the carrier gas brazing process, if the diffusion of the skin material is vigorous, Zn in the core material will evaporate therefrom, so that the remaining Zn amount in the fins will be insufficient to attain the sacrificial anodic effect expected.
In addition, there has presently been suggested a novel method for manufacturing a fin material for an aluminum alloy heat exchanger by the use of the brazing technique (Japanese Patent Application (OPI) No. 215,729/1985), and according to this new method, the fin material can be manufactured in which an aluminum alloy core material containing predetermined amounts of Mn, Mg and Zr is clad with a skin material.
In the method disclosed in the gazette of Japanese Patent Application (OPI) No. 215,729/1985 above referred to, it is contemplated to heighten the sagging resistance by adding Zr to the fin material and carrying out one annealing step as an intermediate treatment of a predetermined cold rolling. In recent years, however, a required level of the high-temperature sagging resistance of the brazing sheets is getting higher, as the fin material is thinned, and therefore the above suggested method is too poor to satisfy the enhanced requirements.
Further, in the non-corrosive flux brazing process or the carrier gas brazing process, the Zn component is added to the core material, as described above, but the thus obtained conventional brazing sheets are low in high-temperature sagging resistance, and the requirement for rendering the fin material thin, cannot be satisfied.